Contact & Non-Contact Forces (Oxford AQA IGCSE Physics)

Contact & Non-Contact Forces

What is a force?

• A force is defined as:

A push or a pull that acts on an object due to the interaction with another object

• Forces acting on objects can change the object's

  • Speed

  • Direction

  • Shape

• For example:

  • The force exerted by an engine (thrust) can increase the speed of a vehicle

  • A comet’s direction can be affected by gravitational attraction

  • A spring can have its shape changed by the force from a heavy load

Effects of forces

• All forces can be categorised into one of two types:

  • Contact forces

  • Non-contact forces

Contact forces

• A contact force is defined as:

A force which acts between objects that are physically touching

• Friction:

  • is a force that opposes motion

  • occurs when the surfaces of objects rub against each other

• Drag:

  • is a type of friction

  • occurs when particles in a fluid (a gas or a liquid) collide with an object moving through a fluid

• Air resistance:

  • is a type of friction and a type of drag

  • occurs when air particles collide with an object moving through air

• Tension:

  • occurs when a pair of forces pull from opposite ends of an object

• Normal contact force:

  • is sometimes called the reaction force

  • occurs when objects are supported by a surface

Friction and air resistance

Normal contact force

Non-contact forces

• A non-contact force is defined as:

A force which acts at a distance, without any contact between bodies, due to the action of a field

• All non-contact forces act at a distance due to the presence of fields

• Weight:

  • is the force that acts on an object with mass when placed in a gravitational field

• Electrostatic force:

  • is a force experienced by charged objects when placed in an electric field

    • For example, the attraction between a proton and an electron

• Magnetic force:

  • is a force experienced by a magnetic pole when placed in a magnetic field

    • For example, the attraction between the North and South poles of magnets

Non-contact forces acting on objects

Friction

• Friction is a force that opposes the motion of an object

  • Friction slows down the motion of the object

  • Friction causes heating in the objects

• Friction between solid surfaces is caused by imperfections in the surfaces of the objects moving over one another 

Friction between solid surfaces

• Gases and liquids are known as fluids

  • Fluids are different to solids because the particles in fluids can move around

• Friction acts on objects moving through fluids as the particles collide with the object 

  • This type of friction is called drag

• Air resistance is a type of friction that slows the motion of an object moving specifically through air

• Particles bump into the object as it moves through the air

  • As a result, the object heats up due to the work done against the frictional forces

Air resistance of a rocket through the atmosphere